This invention relates broadly to an improvement of electrolytic activating solutions useful for treating zirconium and zirconium alloys (hereinafter collectively referred to as zirconium) prior to electroplating a plateable metal layer on the zirconium and to a related electroplating process for zirconium.
It is well-known that zirconium and zirconium-base alloys and particularly those alloys used in nuclear reactors as cladding materials or in the fabrication of pressure tubes, have limited applications due to the corroding action of the coolants, generally, pressurized water, carbon dioxide, terphenyl or steam. Furthermore, the fabrication of such alloys is difficult and costly because of the rapid corrosion in air at temperatures about 800.degree. to 900.degree. C.
A zirconium oxide film of low ductility is formed on the surface of zirconium during fabrication. This film has a tendency to thicken and to eventually scale off. Simultaneously with the thickening and scaling off of the zirconium oxide film, oxygen penetrates into the subjacent metal and causes these areas to become brittle. This phenomenum is even more pronounced as the temperature of the zirconium is increased.
To protect zirconium against such corrosion, it has been proposed that it be protected by coatings for which various coating processes have been developed. Different metals have been tried as a coating material including aluminum, copper, nickel, and iron. However, the deposition techniques have been inefficient, particularly where the metals are submitted to a high temperature during either utilization or transformation. The primary defects of the prior deposition techniques include unevenness or great thickness of the coating and the lack of adherence or adhesion of the coating, particularly when hot. Also the maximum temperature at which the coatings may be used may be inadequate and limited because of the diffusion between the zirconium and the coating itself or due to the formation of fusible eutectics or both.
The depositing of metal layers on zirconium pieces had been tried by various processes with limited success. The deposits resulting from the electroplating comprise spaced apart modules of metal which require the coating to be thick if it is to be continuous due to the progressive surface increase of the modules and their bonding. Moreover, even when a continuous outer coating was obtained, the coatings had inherent defects of adhesion particularly at high temperatures. Accordingly these electrolytically plated zirconium pieces have a tendency to blister and thus fail to protect the zirconium, particularly when the zirconium is subjected to deformation.
A process in the plating of zirconium with chrome is disclosed in U.S. Pat. No. 3,502,549 in which the zirconium is electrolyzed in an aqueous electroyltic bath of from 400 to 500 grams per liter of chromium trioxide, 10 to 40 grams per liter of strontium sulfate and 30 to 80 grams per liter of K.sub.2 SiF.sub.6 with a current density of from 5 to 40 A./dm.sup.2 in the presence of a lead-base alloy anode, stirring the bath and maintaining the temperature of the bath between 10.degree. and 30.degree. C.
U.S. Pat. No. 3,368,951 discloses a nickel plating process for a zirconium or thorium substrate from a nickel plating bath which is an aqueous solution consisting essentially of from about 20 to about 50 grams per liter nickel sulfate, from about 6 to about 12 grams per liter zirconium sulfate, from about 10 to about 30 grams per liter sodium hypophosphate, from about 10 to about 30 grams per liter sodium acetate and from about 10 to about 30 per liter sodium citrate. Following cleaning the metal is immersed in the metal plating bath which is maintained between 85.degree. and 100.degree. C with application of a D.C. voltage of from about 1 to about 5 volts between an anode and the zirconium or thorium substrate.
In the Journal of Electrochemical Society, Volume 100, page 289 (1953), there is a disclosure that a molar ratio of 1.2 to 4.1 of NH.sub.4 F/HF provided good adhesion of electroplatings on zirconium and this is believed due to the formation of zirconium hydrides to give electronic conductivity required for plating. An aqueous activating solution of 29 grams of KF and 50 grams of HF was reported to give good results in Energia Nucleare, Volume 11, page 505 (1964), In Memoires Scientifique Rev. Metallurg., Volume 63, page 1 (1966), there is a report that Zircaloy can be activated anodically for copper and nickel platings in an aqueous bath of 50% HCL, 10% Glycerine, 0.5% butanediol and a wetting agent. Another anhydrous eutectic solution is disclosed comprised of 41% LiCl, 49% KCl, and 10% CuCl.sub.2 at 400.degree. to 500.degree. C.
While the foregoing have produced coatings on zirconium, it has remained desirable to achieve even more improved electroplating processes for zirconium.